1. Field of the Invention
The present invention generally relates to an image data storage system or apparatus that has a plurality of storage means for storing the image data. More specifically, the present invention relates to such an image data storage system or apparatus that has a plurality of storage means for storing the image data to be used in a digital copying machine.
2. Description of the Related Art
As one of today's known digital copiers, there is a digital copier that has a hard disk drive (hereinafter referred to as HDD) as a means for storing image data. A HDD as such has been utilized for a function that stores the image data once and then uses the data, e.g., an electronic sorting function, an image registration function, etc. In the electronic sorting function, original image data are obtained by scanning a plurality of pages or sheets, then the obtained original image data sets are stored into a HDD and thereafter each image data set in the order of its corresponding page number is read out from the HDD for the purpose of making printouts. This function enables delivery of sorted copier-paper printouts without a more-conventional, sorting-hardware mechanism, which has a plurality of sorting bins. On the other hand, in the image registration function, a plurality of form images (sets) as the registration images are stored into the HDD and thereafter printouts can be made on an on-demand basis, eliminating the necessity of a scanning process that would otherwise be required each time when additional copies are made.
Nowadays, we have seen the necessity of using a plurality of HDDs in parallel for storing the image data in order to keep up with the requirements of higher speeds in print-out performance while using the electronic sorting function, not to speak of the requirements of high speeds due to the increases in amount of information to be handled, which come from recent years preferences for high resolution images. For instance, an A4-sized and 1,200-dpi image amounts to about 17 MByte of data, when each pixel is considered to occupy 1 bit. If a print-out performance of 120 pages per minute are required, the required data transfer rate of the HDD would be about 34 MBytes per second, as is obtained by the calculation of: (17 MBytes×120)/60 sec.=34 MBytes/sec. However, at present, a typical data transfer rate of a reasonable HDD is about 20 MBytes/sec., which is unable to satisfy the above data transfer rate requirement of 34 Mbytes per sec. Thus, in order to satisfy the above print-out speed requirement, two HDDs simultaneously operated in parallel are utilized, which achieves a data transfer rate of about 40 Mbytes per sec. There is another approach to improve the data access speed without the use of a plurality of HDDs: Japanese unexamined patent publication (KOKAI) No. 2000-32243 shows a copier apparatus utilizing a HDD, with a consideration of data access rates which vary depending on various storage areas of the HD (hard disk), which seems to be able to temporarily improve the data access speed of the same HD.
On the other hand, registered document-form images or those in conjunction with the image registration function that are preserved as stored data in a hard disk tend to be utilized a number of times. Therefore, what is important is the reliability, i.e., the prevention of loss of correct data due to malfunctions, etc., rather than the print-out speeds. In this respect, the aforementioned Japanese unexamined patent publication (KOKAI) No. 2000-32243 does not seem to teach any reliability considerations on its preserved data in its HDD.